If you have older ANSI PST files, you can convert them to Unicode PST to take advantage of the increased storage capacity and other benefits. Here are the steps:
Open Microsoft Outlook and create a new Unicode PST file. Import data from the ANSI PST file into the new Unicode PST file. Ensure that all data is accurately transferred before deleting the old ANSI PST file.
For detailed instructions, you can refer to Microsoft’s official guide on converting PST files.